aura: Clean up compositor initialization/destruction.

content/browser/aura/gpu_process_transport_factory.cc

Index: content/browser/aura/gpu_process_transport_factory.cc
diff --git a/content/browser/aura/gpu_process_transport_factory.cc b/content/browser/aura/gpu_process_transport_factory.cc
index 6ac45f7665d22277ed3448c62c0fc14a1b32f9dc..23834855c799e23d9a76494a9faad35efd1fd6d2 100644
--- a/content/browser/aura/gpu_process_transport_factory.cc
+++ b/content/browser/aura/gpu_process_transport_factory.cc
@@ -319,6 +319,8 @@ void GpuProcessTransportFactory::RemoveCompositor(ui::Compositor* compositor) {
     gl_helper_.reset();
 }
 
+bool GpuProcessTransportFactory::UsesTestContexts() { return false; }
+
 ui::ContextFactory* GpuProcessTransportFactory::AsContextFactory() {
   return this;
 }
@@ -405,14 +407,15 @@ GpuProcessTransportFactory::OffscreenContextProviderForMainThread() {
         base::Bind(&GpuProcessTransportFactory::
                        CreateOffscreenCommandBufferContext,
                    base::Unretained(this)));
-    shared_contexts_main_thread_->SetLostContextCallback(base::Bind(
-        &GpuProcessTransportFactory::
-            OnLostMainThreadSharedContextInsideCallback,
-        callback_factory_.GetWeakPtr()));
-
-    if (shared_contexts_main_thread_.get() &&
-        !shared_contexts_main_thread_->BindToCurrentThread())
-      shared_contexts_main_thread_ = NULL;
+    if (shared_contexts_main_thread_) {
+      shared_contexts_main_thread_->SetLostContextCallback(base::Bind(
+          &GpuProcessTransportFactory::
+              OnLostMainThreadSharedContextInsideCallback,
+          callback_factory_.GetWeakPtr()));
+
+      if (!shared_contexts_main_thread_->BindToCurrentThread())
+        shared_contexts_main_thread_ = NULL;
+    }
   }
   return shared_contexts_main_thread_;
 }